1. Field of the Invention
The present invention relates to a permanent-magnet (PM) stepping motor characterized by the relationship between a magnet diameter of a rotor and an outer diameter of a stator yoke.
2. Description of Related Art
A permanent-magnet (PM) stepping motor is disclosed in Japanese Patent Application Laid-open No. 6-105526. This PM stepping motor is a claw-pole stepping motor. This PM stepping motor includes a first stator and a second stator which has the same structure as that of the first stator. The first stator is constructed such that a bobbin is wound with coils and is contained in a combined structure of an outside yoke and an inside yoke. The first stator and the second stator are stacked in an axial direction, thereby forming a stator. The stator surrounds a rotor which is rotatable with respect to the stator. The rotor is constructed with a shaft, a rotor sleeve, and magnets (permanent magnets) formed by magnetizing plural portions at the outer circumference thereof. According to the invention disclosed in Japanese Patent Application Laid-open No. 6-105526, the outer diameter “d” of the magnet of the rotor and the outer diameter “D” of the stator yoke is set so that the ratio of “d/D” is smaller than 0.4.
In a PM stepping motor having such a structure, rotation and stop are controlled by a relationship between magnetic force of the rotor magnet and magnetic force generated at the stator core by excitation. In this regard, balance of the magnetic force of the rotor magnet and the magnetic force generated at the stator core is an important design consideration. As the permanent magnet, a rare-earth magnet is generally used due to its high magnetic force, but a ferrite magnet is advantageous in view of material cost.